The boiler and the surrounding chamber are conventionally installed in a hall which is at least partially made of concrete and whose main function is to provide protection against external shocks such as falling aircraft. The hall is a part of a building which stands on a raft and which includes other halls for housing various items of equipment. The term "nuclear hall" is used herein to designate the assembly comprising the boiler, its confinement chamber, and the hall which contains them.
It is desireable to hold the boiler and its confinement chamber in such a manner relative to the concrete structure of the hall that the integrity of the boiler is not at risk from horizontal movement which an earthquake could apply thereto. If possible, the boiler should also continue to operate. This means that each component of the boiler must not be subjected to greater accelerations and movements than those for which it was designed. An acceptable level of acceleration could be obtained by limiting the horizontal forces which can be transmited by the structure to the boiler and the confinement chamber, in other words by allowing the assembly to move horizontally in a substantially unhindered manner in the event of an earthquake. Unfortunately such freedom of movement would mean that the boiler would occupy a random position at the end of the earthquake, with the likelihood of its being too far removed from its initial position for continued operation of the nuclear station to be possible.
Since such freedom of movement cannot be tolerated, it seems necessary to provide the nuclear hall with retaining means for limiting the movement of the boiler relative to the concrete structure during an earthquake to a fraction of what it would otherwise be. Unfortunately, the presence of conventional retaining means has the effect of applying accelerations to the boiler which are not just equal to those applied to the concrete structure, but which are much higher. It turns out that during an earthquake, the random movements of the ground (especially the oscillating movements) together with the corresponding accelerations are modified and generally amplified by buildings and other ground supported structures. This is because said buildings and structures are not perfectly rigid, but are capable of elastic deformation.
This effect is particularly true of a nuclear hall: its concrete structure provides a first stage of amplification of accelerations, which when transmitted to the nuclear boiler are further amplified by the metal structure of the boiler, in such a manner that boiler components are subjected to accelerations derived from those of the ground, but after passing through two successive stages of amplification. The first stage may amplify by more than four times for some kinds of concrete structure and for frequencies in the range 2 to 20 Hz. Likewise, the second stage may amplify by more than four times for some metal structures and for frequencies in the range 2 to 20 Hz.
The problem of providing retaining means is further complicated by the various thermal expansions to which the chamber and other components are subject during normal operation of the boiler. Further, in the event of an accidental leak of cooling fluid, the chamber will expand due to the increase in internal pressure and temperature, and it is under these conditions that it must stand up to the highest forces. The problem is thus to dispose the boiler and the confinement chamber inside the concrete structure in such a manner as to ensure both the integrity or non-disruption of the boiler in the event of an earthquake and the integrity of the chamber in the event of a leak of primary cooling fluid. Finally it should be observed that the forces and the energy likely to be applied to the boiler during an earthquake are very high, with forces of around 9000 tons and an energy of about 10.sup.9 J for a boiler weighing 4,500 tons being subjected to 100 oscillations of 10 cm amplitude at a frequency in the range 3 to 15 Hz.
One solution which the present inventors have already proposed to this problem consists in making the base of the steam production block in such a manner that it can be anchored to the raft of the concrete structure sufficiently rigidly to minimise relative movement therebetween. The confinement chamber is then connected to the raft in a manner that does not hinder its dilation.
The raft of the concrete structure must then be thick enough to stand up to the forces to which it is subjected during an earthquake. When the station is not a low power station, the raft must be very thick since the mass of the steam production block is measured in thousands of tons and its height in tens of meters. The bending moment applied to the raft is proportional to the mass of the block multiplied by the height of its center of gravity, and the anchoring forces on the raft are proportinal to the moment and inversely proportional to the transverse dimensions of the base. The cost of the raft starts to become an excessive fraction of the total installed cost of the nuclear boiler.
Further, again because of the mass and the height of the boiler, the mass and the cost of the supporting structure for the boiler are both high, since the structure must be sufficiently rigid from its base to avoid excessive bending in the event of an earthquake, where the bending moment applied to the base of the structure is naturally the same as the moment applied to the raft.
Preferred embodiments of the present invention provide a nuclear hall capable of preventing excessive acceleration being applied to a nuclear boiler in the event of an earthquake, without excessively increasing the cost of the concrete raft on which the hall is built nor the cost of the metal structure placed on the raft to support the boiler. Further, such preferred embodiments do not hinder dilation of the confinement chamber surrounding the boiler, nor do they hinder dilation of the boiler itself.